The present invention relates to devices for sensing moisture content in soils and to thereby control the supply of water to desired areas in agricultural and horticultural situations.
U.S. Pat. No. 4,513,608 discloses one form of sensing device having two separate porous zones with a pair of electrodes in each zone thereby forming a current path through the respective porous zones. In use, the device is placed in an in ground position such that ground moisture may move into or from each of the porous zones. The change in electrical resistance in the current path, through the respective porous zones, is then used to indicate ground moisture levels. This device has proved to be accurate and reliable in use, but does suffer from the disadvantage that its manufacture is relatively time consuming and expensive furthermore expensive cermanic materials must be used to maintain accurately uniform porosity levels in the respective porous zones.
There have been other moisture level sensors proposed using various techniques, such as electrical resistance sensing or impedance or capacitance sensing, as the means of moisture level detection, these moisture level sensors introduce their own problems and complications, especially when working in such an hostile environment physically, chemically and biologically as the soil in practical or commercial outdoor applications. Thermal diffusivity has also been proposed as a means of detection and this does have some practical advantages, although previously proposed devices using this technique have certain disadvantages when in use in a practical commercial environment (as opposed to laboratory situations).
The basis of using thermal diffusivity as a detection means is as follows. Transmission of heat energy through a dry porous material is generally poor because the particles only make point contacts where they abut, hence the area for heat flow is necessarily small. As moisture content increases, capillary meniscus form at the points of contact considerably increasing the transfer of heat between particles, as water is a much better conductor of heat than air. The thermal conductivity of the composite will increase significantly with the increase of water content.
As the moisture content increases, air is displaced from the pores and as water has a higher specific heat, the specific heat of the composite also increases.
In a porous material, the rate of dissipation of heat depends on the thermal conductivity, the specific heat and the density. A constant known as "Diffusivity" has been used to relate these properties in an isotropic homogeneous material, when its thermal conductivity does not depend on temperature.
As indicated above, the relationship between Thermal Diffusivity of a porous material and its moisture content has long been used in laboratory situations to monitor moisture content of soils. In an early example, a copper wire heating coil was used, and by measuring resistance change in the same coil over a given heating period, a temperature relationship was established with its diffusivity, (Baver & Shaw 1944 U.S. Pat. No. 2,362,344).
In order to improve robustness of the element and stabilise thermal interface, a separate thermistor type temperature sensor was introduced and encapsulated with the copper coil heater in a standardised medium within a porous ceramic housing (Richards 1955 U.S. Pat. No. 2,718,141).
Although relatively satisfactory in research type applications, that mode of operation could not be applied to automatic irrigation control. An attempt was made based on duplication of devices, with the heat source and temperature sensor being combined in the form of a positive coefficient thermistor coupled to a current sensing relay (Hassenbeck 1971 U.S. Pat. No. 3,553,481). It is unlikely that this would be practical, due to magnitude of the resistance change required to provided minimum current change for reliable relay operation.
A further modification was made using a single device and combining a single thermistor embedded within a porous media of glass beads (Hassenbeck 1974 U.S. Pat. No. 3,847,351). This combination, in conjunction with current threshold sensing, has been employed commercially, although its lack of environmental temperature compensation and the high heat release it required probably limited its accuracy and reliability.
Determination of temperature rise by deducting initial temperature from final temperature after a known amount of heat has been added, change in diffusivity has been calculated in fluids (Bowman 1975 U.S. Pat. No. 4,059,982). This principle was combined with a separate heat source and used with a diode temperature detector in conjunction with timing, storage, combining, comparing and controlling means (Neal 1977 U.S. Pat. No. 4,197,866).
A similar but more specific device is disclosed in U.S. Pat. No. 4,886,088 to Ryokai & Wakabayashi, using energy source, energy level detection and control means.
Most previous attempts to employ thermal diffusivity as a detection means in soil moisture sensing have lacked positive drift free calibration, have not had a matric tension based response, have not had a positive temperature reference or have employed complex detection control means or some combination of the aforementioned.